1. Field of the Invention
The present invention relates to an image forming apparatus configured to fix a developing material to a sheet by using a heat-fixing device and, in particular, to discharge control of the sheet which has passed through the heat-fixing device in such an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus employing electrophotography, represented by a laser beam printer and a copying machine, a toner image transferred onto a sheet is fixed to the sheet by using a heat-fixing device and the sheet is discharged onto a discharge tray. As a result of the recent increase in speed in image forming apparatuses, the sheet heated by the fixing device is discharged onto the discharge tray before the sheet has been sufficiently cooled. Thus, the sheets stacked on the discharge tray pile up without the toner thereon being sufficiently cooled, resulting in a drawback that the sheets stick to each other. In view of this, Japanese Patent Application Laid-Open No. 2002-268305 discusses a technique in which before a sheet is stacked on discharge tray, the sheet is temporarily stopped during the course of discharge, and a part thereof is exposed to the exterior of the apparatus or cooled by cooling air from a cooling apparatus to thereby sufficiently cool the toner thereon, subsequently the conveyance resumes and the sheet is stacked on the discharge tray. In this way, the sheets are prevented from sticking to each other.
However, in the case where the conveyance of the sheet is temporarily stopped at the discharge unit to cool the sheet, the leading edge portion of the sheet at rest trails down, so that the sheet is stopped while in contact with the preceding sheet stacked on the discharge tray. FIG. 5 is a schematic diagram illustrating how the sheet conveyance is temporarily stopped at the discharge unit of a printer.
When the leading edge of a sheet 14 to be discharged onto a discharge tray 15 is detected by a discharge sensor 13, the conveyance of the sheet being discharged with a predetermined timing is temporarily stopped. In this case, the sheet 14 stops in the state as illustrated in FIG. 5, and the leading edge of the sheet 14 comes into contact with a sheet 16 already stacked on the discharge tray 15 at a position 17.
When again conveying the sheet 14 temporarily kept at rest, there is exerted a maximum static friction force at the position 17 where the sheet 14 is held in contact with the sheet 16. This maximum static friction force is a resistance force larger than the dynamic friction force that would be exerted when discharging the sheet without temporarily stopping the conveyance. As a result, there is a possibility of the leading edge of the sheet 14 being caught by the sheet 16 discharged and stacked on the discharge tray 15, resulting in a defective stacking of the sheet 14; or there is a possibility of the discharge sensor 13 erroneously detecting a fully stacked state when in reality the stacking tray is not fully stacked.
It might be possible to temporarily stop the conveyance before the leading edge of the sheet 14 comes into contact with the sheet 16; in this case, however, the major portion of the sheet 14 would remain inside the apparatus, disabling to achieve a sufficient cooling effect.